Cylindrical electrochemical cells have the active components contained in a container that has an open end. This end is closed with seal assembly that is comprised of a plastic seal member, a cover terminal and a current collector. The terminal contacts one of the cell electrodes via a current collector. The current collector is generally a nail-like piece that passes through the plastic seal to contact either the anode or cathode.
The seal assembly is intended to perform two somewhat conflicting functions. The first function is to seal the cell so the components do not leak out under conditions of normal use and so the components are not contaminated by contact with air and moisture. The second function is to vent the cell when the internal pressure exceeds a predetermined value.
To perform the first function the seal assembly is made of a synthetic material that can act as a barrier to air and moisture and will not corrode from contact with air, moisture and the cell components. The material must also be strong enough to maintain its sealing function after it has been physically abused such as by dropping or exposure to vibration and/or subjected to extreme variations in temperature and/or humidity.
To perform the venting function, a vent means is built into the seal assembly. By designing the cell to vent under abuse conditions, violent rupturing of the cell can be avoided. Some common vent means include an opening sealed with a meltable or blow-out plug and a scored area in the surface of the sealing member. The scored area will provide a thinner and consequently weaker portion of the surface of the seal member to provide the venting function.
Unfortunately, it is difficult to manufacture seals that perform both functions reliably. The typical seal members are made of synthetic materials by molding. During the molding operation, the synthetic materials may not flow properly to fill the mold, and the resultant seal member will not conform to the necessary specifications. One of the most common defects in the seals is known as hub splitting, i.e. cracks or splits are present in the seal where the current collector passes through. This defect can occur if a weak spot is formed in the hub during molding and the nail-like current collector is inserted via an interference fit through the hub. Contacting the seal with a crazing agent, such as potassium hydroxide, increases the stress on the seal's hub thereby increasing the likelihood that a defective hub will eventually split. If this defect is present, the electrolyte, such as KOH, can migrate from the cell's interior through the hub split to the cell's exterior.
A contributing factor to the potential for hub splitting is the design of the venting means in the seal. When the vent means is provided by molding the seal with areas of different thicknesses, the synthetic material must flow from thin to thick areas. However, this flow may not be uniform since the different thicknesses will create high pressure gradients during filling of the mold. These gradients adversely impact the ability of the molten plastic to pack into the mold thereby weakening the mechanical properties of the molded part. Thus, some portions of the seal may cool and cure prematurely resulting in a weak spot.
The design of the vent in the seals may itself lower the reliability of the vent. Many seals are made with a thin ventable portion that has a uniform cross-sectional thickness. When the internal cell pressure increases, the intent is that the thin ventable portion will burst. However, many times the vent will balloon rather than burst. Thus, the pressure is not relieved.
In view of these disadvantages, there is a desire for a seal that can be made without the problems of nonuniform packing of plastic and that will reliably seal the cell under normal conditions and will reliably vent the cell when required.